This invention relates to scroll-type rotating machines, such as scroll compressors as used in air conditioning and refrigeration. The invention is more particularly directed to the achievement of superior finishing and surface smoothness in surfaces of moving parts that mate together in a scroll machine.
Scroll type rotary machines are advantageously used to compress or pump a gas. These machines typically have two scroll members that face each other, each formed of a generally circular base plate and a spiral or involute wrap that is generally erect with respect to the base. That is, the wrap has side walls that are parallel to the axis of the scroll member. The scroll members maintain a fixed azimuth relative to one another, but are radially offset so that one can revolve about the other in an orbiting fashion. Relative orbiting motion is typically achieved by holding one scroll member fixed in a housing, and orbiting the other by rotating an eccentric shaft while holding the orbiting scroll member with an anti-rotation device, such as an Oldham's ring. Orbiting motion can also be achieved by rotating both scroll members on parallel, offset axes.
The walls of the involute wraps define crescent-shaped volumes which become smaller and smaller, and which move from the outside to the center of the mating scrolls with motion of the orbiting scroll element. A compressor fluid, such as a refrigerant gas, can be introduced at the periphery of the spiral wraps, and this fluid is compressed as it is moved under the orbiting motion of the device. The compressed fluid is then discharged at center. By introducing a compressed fluid into the center of the device and permitting it to expand, the scroll machine can be used as a motor.
In order to achieve good efficiency characteristics, mating surfaces of the scroll elements must be as regular and smooth as possible, both to minimize frictional losses and to minimize gas leakage from the crescent-shaped volumes. Flank leakage can occur across the axial line where the walls of the two wraps are tangent, and tip leakage can occur where the tip of one scroll wrap contacts the flat base surface of the mating scroll element. Generally, tip leakage is a greater source of loss and inefficiency than flank leakage. Also, imperfections in flatness of the base surface of the scroll elements are responsible for thrust friction losses.
Machining a scroll element so that the base wall is flat and the walls of the wrap are axially erect spirals is extremely difficult to accomplish. The presence of the wrap itself prevents standard grinding and machining techniques from being employed for this end, because the operative surfaces to be finished are recessed below the tip of the scroll wrap.
There has been no simple technique proposed previously to finish and flatten the scroll element surfaces so as to minimize leakage and frictional losses.